An image forming apparatus having a current measurement system for measuring an electrical current in a transfer unit is known. It measures an output current in a condition that the transfer unit is not in operation and that in a different condition. Then, it performs a comparison between the measured output currents. It determines whether the transfer unit is provided, or whether an abnormal operation in switching between standby and contact positions occurs based on the result of the comparison.
In an image forming apparatus, an improper exposure may occur. In this case, a photosensitive body does not get sufficient exposure. However, in the above known image forming apparatus, such an improper exposure is not seriously discussed. It only discloses how to determine whether the transfer unit is provided, or whether an abnormal operation occurs in switching between the standby and contact positions.